Early work relating to network-based wireless location systems is described in commonly assigned U.S. Pat. No. 5,327,144 “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. Further enhancements of the system disclosed in the '144 patent are disclosed in commonly assigned U.S. Pat. No. 5,608,410 “System for Locating a Source of Bursty Transmissions.”
Modification of mobile transmissions to improve the location capability of a network-based wireless location system was previously disclosed in commonly assigned U.S. patents:                U.S. Pat. No. 6,097,336; “Method for Improving the Accuracy of a Wireless Location System”        U.S. Pat. No. 6,115,599; “Directed Retry Method for use in a Wireless Location System”        U.S. Pat. No. 6,463,290; “Mobile-assisted network-based techniques for improving accuracy of wireless location system”        U.S. Pat. No. 6,519,465; “Modified Transmission Method for Improving Accuracy for E-911 Calls”        
Use of parallel or successive location estimates to fulfill the expected or demanded location quality of service has been previously disclosed in the following commonly assigned US patents:                U.S. Pat. No. 6,503,428; “Multiple Pass Location Processing”        U.S. Pat. No. 6,873,290; “Multiple Pass Location Processor”        U.S. Pat. No. 7,023,383; “Multiple Pass Location Processor”        
Third generation mobile services such as Universal Mobile Telecommunications System (UMTS), a wide-band code division multiple access (CDMA) modulated air interface, are being deployed in many areas of the world. Until deployment is more widespread, coverage may not be complete and thus in some coverage areas third generation services may not be available or a mix of third generation and legacy services may be provided. For example, one coverage area may only provide Global System for Mobile communications (GSM) services, while an adjacent coverage area may provide mixed UMTS/GPS services. A mobile terminal equipped with both UMTS and GSM technology may thus provide a mobile user with seamless mobile service when traveling between coverage areas. Such dual-mode mobile terminals may require a handoff mechanism between the UMTS and GSM technologies. For example, if the user has established a voice call using UMTS technology, and then moves outside of UMTS coverage, it is desirable that the voice call be handed over to the GSM network without perceived interruptions. Such a handover mechanism is provided by the Inter-Radio Access Technologies (Inter-RAT) handover technique. The Inter-RAT capability in the context of dual mode UMTS and GSM mobile devices is supported by 3rd Generation Partnership Project (3GPP) standards.
Diversity of radio access technologies (e.g., AMPS, TDMA, GSM, iDEN, CDMA, W-CDMA, and OFDM) may provide certain advantages and disadvantages in locating mobile devices. For example, the diverse characteristics provided by a second radio network type, such as antenna placement or downtilt, may provide a more accurate network-based location determination. A second radio network type may also mitigate problems encountered using a first radio network. Using multiple radio networks for network-based location may also take advantage of the differing topology of the radio networks. For example, one network may have larger or smaller cell-site spacing then the other network. In some cases, the second radio network may provide a better geometric dilution of precision (G-DOP) than the first. Additionally, a combination of both networks may provide a better G-DOP than either network alone.
Previous location determination systems, such as the TruePosition Location Platform (also known as the Wireless Location System (WLS)) supported location determination using the Inter-RAT handover technique using the capability of dual mode UMTS and GSM mobile devices and the Inter-RAT feature. Location determination using mixed UMTS-GSM networks have been used to provide low-rate location services (such as emergency services, e.g. 9-1-1, 1-1-2, and 9-9-9 services) for both idle mode and mid-call scenarios without the need to install more advanced dual-mode GSM and UMTS Location Measurement Unit (LMU) networks.
Deployment of the Inter-RAT handover technique typically requires that the wireless operator's UMTS Radio Access Network (RAN) vendor support a Inter-RAT handover feature in its Mobile Switching Center (MSC) software. This handover capability may be used to support location determination for UMTS mobile stations in dual mode UMTS/GSM networks using time difference of arrival (TDOA) or TDOA and angle of arrival (AoA) location techniques based on LMUs installed for AMPS, TDMA and GSM locating. For example, dual mode GSM and UMTS air interface support is typically available for new UMTS mobile devices due to the limited deployment of UMTS networks as discussed above. Some carriers also deploy mobile devices that support UMTS (or other 3G technologies) and GSM for international roaming.
However, the existing multi-mode systems (such as the UMTS/GSM dual mode scenario with handover capability) do not utilize radio technology diversity for location determination. In the UMTS/GSM example, the handover mechanism may be used to direct the mobile device to use a second radio technology when a first technology is not available. However, existing systems do not support location determination using, for example, a first location using UMTS followed by a second location using GSM. It would also be advantageous to first perform an analysis of the first location determination to determined if a second location determination may improve upon the results of the first location determination. Furthermore, it would be advantageous to utilize both the control channel and traffic channel to provide further diversity opportunities. There is a thus need to enable greater use of the multi-mode mobile device capabilities and the Inter-RAT functionality of current and developing wireless communications networks to provide improved location estimation capabilities.